The present invention relates in general to an arrangement for measuring differences of electrical potentials, and in particular to measuring potential differences in second-class conductors, such as ionized liquids.
In boundary layers or interfaces between solid bodies such as metals, ion-selective glasses, membranes and also liquids on the one hand, and ion-containing liquids generally called second-class conductors on the other hand, potential differences frequently occur, and the measurement of such differences is of particular interest. Interfaces of this kind in the following description will be referred to as potential-generating layers.
For example, known are ion-selective electrodes provided with membranes of synthetic material acting as potential-generating layers (Cammann "Das Arbeiten mit ionenselektiven Elektroden", Springer, Berlin, 1973), with which, upon immersion in an ion-containing solution, potential differences occur which correspond to the activity of ions under measurement. In this manner a large number of ions can be determined.
Such known electric measuring means employed for measuring the potential difference cause however a serious problem, inasmuch as the metals which connect amplifying or indicating devices to respective measuring electrodes, when immersed in the ion-containing solutions, produce themselves potential differences, and a chain of potentials will result due to which an unambiguous correlation between the measured potential difference and that of the potential generator itself, or the calculation of the resulting interfering potentials, is not always possible. This problem represents a particularly disturbing limitation of the measuring accuracy, because the interfering potentials always depend on the particular substance under measurement, and for example in biological measuring substances they can hardly be predetermined as far as their magnitude and time behavior are concerned.
For instance, the drift and the absolute value of potential differences cannot be determined in advance in many cases when calomel electrodes or also Ag/AgCl electrodes are used in biological objects.
In the application of liquid compounds between the indicating device and the measuring probe, applied for example by means of agar pipettes, frits or capillaries, does not result in any satisfactory solution of the problem, because such solutions in most cases become quickly contaminated, produce parasitic diffusion potentials, or contaminate the measured substance.